The present invention relates to the preparation of plastics articles by vacuum forming or thermoforming.
Resinous compositions based on copolymers of a styrenic monomer and an ethylenically unsaturated nitrile, in particular styrene/acrylonitrile copolymers (SAN resins), find use in a variety of applications on account of their toughness and resistance to chemicals, sunlight and the weather. Unmodified such copolymers are relatively brittle and to improve their impact strength it has long been known to disperse uniformly throughout a continuous phase of the styrenic monomer/nitrile copolymer a discrete elastomeric phase of an impact modifier. To improve compatibility between the continuous and disperse phases, the impact modifier usually is a graft copolymer comprising a rubbery substrate, such as a poly 1,4-butadiene or copolymer of 1,4-butadiene with either or both of styrene and acrylonitrile, and a grafted superstrate of styrenic monomer/nitrile copolymer. The resulting dispersions are best known in the form of ABS resins.
The resinous compositions based on styrenic monomer/nitrile copolymers, whether modified or unmodified, are used as engineering thermoplastics to form a variety of shaped articles, frequently of thin wall structure, by processes involving elongational flow, such as vacuum forming or thermoforming. For example, such articles as refrigerator linings and boats can be formed by vacuum- or thermo-forming sheets of ABS resin compositions. A problem encountered when forming such articles is that of uneven wall thickness, so that the wall thickness in areas, such as corners, subjected to high levels of deformation is less than in areas, such as long stretches of straight wall, subjected to low levels of deformation. To overcome this problem, and provide articles of sufficient strength and more uniform wall thickness, it has frequently been necessary to increase the thickness of the sheets of resin composition used in the moulding process. This, however, results in the use of increased amounts of resin and is wasteful in that the wall thickness in areas which have been subjected to low levels of deformation is greater than is required in practice.
GB-A-No. 2111514 describes a thermoplastics resin composition suitable for preparing biaxially oriented materials which comprises a base thermoplastics resin and and a thermoplastics resin B compatible therewith having a molecular weight of more than 10.sup.6 and obtained by the polymerisation of at least one vinyl monomer. In the preparation of biaxially oriented materials it is necessary to employ temperatures close to the glass transition temperature of the base resin A and relatively high stress levels (usually at least 2.times.10.sup.6 Pa up to 10.sup.7 Pa). Under these conditions the relaxation times of the resin molecules are long and the moecules remain oriented in the final product to provide it with the desired mechanical properties. If the temperatures employed are too high, for example 30.degree. to 40.degree. C. above the glass transition temperature of the base resin, or the stress levels too low, for example 1.times.10.sup.6 Pa, then the relaxation times of the resin molecules are short and there is insufficient residual orientation to provide the desired balance of physical properties in the final product.
By way of contrast vacuum-forming or thermo-forming operations are usually carried out at temperatures significantly (at least 40.degree. C.) above the glass transition temperature of a base resin and at relatively low stress values (10.sup.6 Pa or below corresponding to the subatmospheric pressure applied). A problem then is, as mentioned above, to achieve reasonably uniform wall thicknesses in moulded articles during the preparation of which significantly different levels of deformation are encountered because of the shape of the articles. Put another way, the elongational viscosity of the moulding composition under the forming conditions should decrease as little as possible with increases in the level of stress.